The continuous emission of greenhouse gases causes global warming, resulting in a series of environmental problems such as glaciers melting, sea level rise and frequent extreme weathers, which will cause serious, widespread and irreversible effects on humans and ecosystems. Methane is the second largest greenhouse gas in the world, its greenhouse effect is 28 times than that of carbon dioxide, its contribution to the atmospheric radiation intensity is more than 18%, and its impact on climate change is more significant than that of carbon dioxide. The anaerobic technology in the water treatment field is widely used worldwide because of low energy consumption, low sludge yield and the production of methane as renewable energy. However, the methane produced by the conventional anaerobic process is often present in the effluent of sewage treatment, and is released from the effluent to the atmosphere, thereby causing the greenhouse effect. At present, wastewater treatment plants have become one of the important emission sources of methane worldwide.
Anaerobic ammonia oxidation (Anammox) and denitrifying anaerobic methane oxidation (DAMO) are the latest discovery in the field of biological nitrogen cycle. DAMO microorganisms include DAMO bacteria and DAMO archaea, which are autotrophic microorganisms using methane as electron donors. DAMO archaea convert nitrate into nitrite, and the DAMO bacteria reduce the nitrite into nitrogen Anammox microorganisms can generate nitrogen and nitrate by means of nitrite and ammonium. Therefore, the coupling of the DAMO microorganisms and the Anammox microorganisms provides a new opportunity for efficient simultaneous nitrogen and methane removal, is also a key link of the carbon and nitrogen cycle, and provides a new idea and choice for methane emission reduction and nitrogen pollution treatment.
However, the DAMO and Anammox microorganisms grow very slowly, with a doubling time from 11 to 30 days, and the accumulation of biomass takes a long time. In the current research, the DAMO and Anammox microorganisms mostly present in the form of flocculent sludge, the settling performance of floc is poor, and the functional microorganisms are easily washout, so the DAMO and Anammox microorganisms are hardly applied to the practice.
Moreover, methane is regarded as a carbon source for the growth of the DAMO microorganisms, while the solubility of the methane in water is less than 22 mg/L under normal temperature and pressure conditions, it is difficult to ensure sufficient methane supply for the growth of the DAMO microorganisms and the denitrification for nitrogen removal, and the gas-liquid mass transfer rate is low, resulting in slow cell growth and slow biosynthesis, which greatly limits the large-scale application of the DAMO microorganisms.